1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer for forming an image using an electrophotographic method and an image forming method.
2. Description of the Related Art
In recent years, a color tandem method has been known as one of image forming methods in image forming apparatuses using an electrophotographic method and represented by copiers and printers. The color tandem method is a method for realizing high-speed full color printing by arranging photoconductive drums of four colors, i.e. black (BK), yellow (Y), magenta (M) and cyan (C) in a row and successively transferring toner images of the respective colors formed (developed) on the respective photoconductive drums to an intermediate transfer member or a transfer member. There exits a prior art document that discloses a technology relating to a correction process for preventing color shift caused due to dimensional errors and mounting errors of mechanical parts such as photoconductive drums in an image forming apparatus employing such a color tandem method.
A surface speed of the photoconductive drum of each color varies due to a rotation variation of a drive motor, pitch unevenness in a transmission gear train for transmitting a drive force of the drive motor, a speed variation caused by eccentric rotation of a gear or a speed variation caused by eccentric rotation of the photoconductive drum itself. Such a surface speed variation of the photoconductive drum repeatedly occurs in a rotational cycle of the photoconductive drum. Position shifts occur among the respective colors due to a variation of fluctuating phases of the respective colors. Accordingly, in the apparatus of the above prior art document, position shifts among the respective colors are prevented by performing a phase synchronization control. The phase synchronization control is a control for detecting phases of the photoconductive drums of the respective colors and independently drive-controlling the respective photoconductive drums so that, with respect to the phase of a specified photoconductive drum as a basis, the other photoconductive drums rotate with predetermined phase differences.
However, if a drive control unit for driving and rotating the respective photoconductive drums and an image forming station for controlling formation of toner images of the respective colors on the surfaces of the respective photoconductive drums are independently constructed, the phase synchronization control as described above is performed by the drive control unit and an image formation control is performed independently of the phases of the respective photoconductive drums in the image forming station. Thus, it has been difficult to recognize phase peaks of the respective photoconductive drums and perform an image forming process in conformity with the phases of the respective photoconductive drums in the image forming station.
Factors of color shift in the image forming apparatus employing the color tandem method are not limited to the dimensional errors and mounting errors of the mechanical parts. For example, image forming apparatuses of recent years are constructed such that a plurality of arithmetic circuits for performing arithmetic processings based on output signals of various sensors and outputting various control signals are provided for faster processing. Generally, these arithmetic circuits are respectively arranged on different substrates to reduce substrate sizes and oscillation circuits as clock signal sources for operations of the respective arithmetic circuits are individually provided on the respective substrates. The oscillation circuits are individually provided on the respective substrates (for the respective arithmetic circuits) because clock transmission quality is problematic due to a high oscillation frequency and unnecessary electromagnetic radiation increases if clock signals generated by the oscillation circuit on a certain substrate are supplied to the other substrates.
However, even if the oscillation circuits are provided on the respective substrates, there is no likelihood that the oscillation frequencies of all the oscillation circuits keep the same relationship since the respective oscillation circuits have tolerances with respect to an ideal oscillation frequency. Such tolerance deviations of the oscillation frequencies present among the oscillation circuits may possibly cause the above color shift. For example, if there is a tolerance deviation in oscillation between an oscillation circuit provided in the drive control unit and the one provided in the image forming station, a relationship between the rotation speeds of the photoconductive drums and exposure processing speeds to the photoconductive drums varies, which may possibly cause the color shift.